Shear Strength of Cohesive Soils and Friction Sleeve Resistance

Transcription

1 Transportation Kentucky Transportation Center Research Report University of Kentucky Year 1974 Shear Strength of Cohesive Soils and Friction Sleeve Resistance Vincent P. Drnevich C. Thomas Gorman Tommy C. Hopkins University of Kentucky Kentucky Department of Highays Kentucky Department of Highays This paper is posted at UKnoledge. researchreports/1447

2 Research Report 394 SHFAR STRENGTH OF COHESIVE SOilS AND FRIGfiON SLEEVE RESISTANCE by V. P. Drnevich Associate Professor of Civil Engineering University of Kentucky C. T. Gorman Research Engineer Assistant and T. C. Hopkins Research Engineer Principal Division of Research Bureau of Highays DEPARTMENT OF TRANSPORTATION Commonealth of Kentucky prepared for presentation to European Symposium on Penetration Testing Stockholm June 5-7, 1974 June 1974

3 SUMMARY Cone penetrations tests ere performed on the silty clays of Kentucky, U.S.A., using a boring rig to push the Dutch, friction sleeve, cone penetrometer. Thin-alled tube samples ere taken from nearby boreholes. For the first four sites, unconfined compression tests and unconsolidated-undrained triaxial tesb ere performed on the samples. For the last four sites, consolidated-undrained triaxial tests ere performed on the samples. A procedure for estimating in situ shear strength from triaxial test stress paths as developed. TESTING PROCEDURE The Dutch friction cone penetrometer ~s adapted to a conventional boring rig as described by Drnevich (1974). Dutch cone pcnelrhlion testing as performed at four highay landslide sites in this study. These sites offered the opportunity to investigate both compacted embankments and foundation soils. Conditions of both full and partial saturation existed, and rock fragments ere encountered in a fe cases. Penetration test results for the four sites arc shon in Figure 1. {Cleveland's ork as performed at naturally occurring Small rock fragments in these rcsidttal soils caused erratic cone resistance at many locations. As a result, the friction sleeve resistance provided the best correlation ith in situ shear strength. In situ shear strength as found to be approximately 8 percent of the friction sleeve sistance, hich confirms the t1ndings of others. CO~ RESiSTANCE I I'm' I FRICTION RESISTANCE I'~'''' I FRlCTION RATIO INTRODUCTION Dutch cone penetration testing as initiated at the University of Kcnt.:ky, USA, in Early efforts by Cleveland (197!) focused on the corrclu1ion of Dutch cone penetration test results ith standard penetration test, soil type identification, laboratory vane shear test, unconfined compression test, and unconsolidated undrained triaxial shear test results. Cleveland's findings indicated that a relationship existed beteen Dutch cone friction sleeve resistance (Begemann, 1953) and shear strength, as measured by unconsolidated undrained triaxial tests. In September 1972, a cooperative effort beteen the Kentucky Department of Transportation and the University of Kentucky as initiated to further assess the capabilities of the Dutch cone penetration test as a means of determining in situ shear strength. Several highay landslide sites ere chosen for investigation. This venture provided the opportunity to expand shear strength correlations to a ide variety of soils. Anng the soils tested ere compacted embankments, residual silty clays, and alluvial deposits of a more silty nature. Conditions of full and partial saturation and normal and over consolidation existed. The results of both studies arc presented herein. BACKGROUND The first attempts at predicting shear strength using the Dutch cone penetrometer involved the correlation of cone resistance, qc, ith shear strength. From bearing capacity theory and equations, an equation relating undrained shear strength, r, to cone resistance, qc, an empirical bearing capacity factor, Nc, and overburden pressure, P, may be derived (Thomas, 1965). This equation is of the form Hoever, P may be neglected, yielding the equation Research correlating qc ith undrained shear strength, as determined by various methods, has yielded values of Nc ranging from 5 to 25 (Sang!erat, 1972). Development of the friction sleeve by Begemann (1953) offered another approach to the determination of undrained shear strength, Begemann (1965) suggested that the value of friction resistance, fs, should be approximately equal to the undrained shear strength. This vie as supported by Tomlinson's (1957) ork ith piles in clay soils. Tomlinson found that pile adhesion as approximately equal to soil cohesion, Cu, for soft clays. Similar research by Vesic (1969) limited this relationship to soils ith undrained shear strengths less than.7 kg/cm2. Experimental correlation of the relation beteen Dutch cone sleeve friction and undrained shear strength as presented by Wesley (1967) and shoed sleeve friction to be slightly higher than undrained shear strength. ~ ' r! Figure 1. (' l GWEN COUNTY BO~O COUHTV -~ FAYETTE COUNTY - lioiirence COUNTY-- - Dutch Cone Penetration Test Results deposits of residual silty clays and alluvial clayey silts.) Thin all Shelby tube samples ere taken near the Dutch cone penetration test holes. These undisturbed samples ere used in subsequent triaxial testing to determine in situ shear strength, Hoever, in the sampling process the in situ total stresses are removed from the sample and some disturbance is inevitable. To overcome this problem, initial in situ conditions ere duplicated for one sample of each set of triaxial tests by consolidating it to the mean in situ effective stress, ae' given by the equation: K varies ith soil origin, soil type, and load history, For a given soil deposit, K varies ith the degree of overconsolidation, hich may be affected by dessication near the surface, ater table fluctuations, and sedimentation and erosion. Test results published by Bishop and Henkel (1957) for compacted embankment soils sho values of K ranging from.35 to.65. The loer values of K pertain to soils having a lo percentage of clay fraction. Generally the Dutch cone tests ere performed in soils having a high percentage of clay fraction. Hence the K values for the compacted embankment soils could be expected to tend toard the higher range of the K values. An estimate of K for the foundation soils encountered as made using test results published by Lambe and Whitman ( 1969) hich gives K as a function of overconsolidation ratio and plasticity index, The range of plasticity index (-2) and overconsolidation ratio (1 4) encountered in these soils yield a range of K from.4 to.8. K as assumed to be,62 for both cases as this value tended toard the higher range for compacted fills and as a median value for the foundation soils. Substituting this value into Equation 1 yields a'c = 3 a'/4 Folloing isotropic consolidation of the laboratory specimen to a' C' the drainage lines ere closed and the sample loaded axially, thereby reproducing undrained failure.

4 An example of a plot of triaxial test data is shon in Figure 2. Note that the stress path method (Simons, 196; Lambe, 1964) is used to sho the continuous stress change during loading. Tests under in situ conditions ere used to determine the shear stress on the failure plane, 1'f. Assuming in situ failure stresses are mobilized hen the in situ stress path intersects the Kf line, the Mohr circle at failure can be defmed from the point of intersection, The values of Tf may be determined from qf and 1/>', Derivation of the equation 1'f = qf cos / is shon in Figure 3. OWON COUNTY ~:~: :: ;:~:~; ::-. ;:; :: ;: ~::~:: ~- ''' ~,,. ''''''' ' Figure 3. Derivation of the Equation rf = qf cos RESULTS!.,~.,,.,~ ~ ---;'cc-'-;,--t,,,,l-;,,,--;;.,,--;,-,t;--tr--.. P' rr, ~ rr; l o lorn' I Figure 2. Typical Triaxial Test Data Index properites of the soils encountered are shon in Table 1. Results of Dutch cone penetration testing and triaxial testing performed on undisturbed samples from these sites are sununarized in Table 2. A statistical analysis of the data produced a regression line ith the equation f 8 = 1.28 rf to describe the data. Work done by Cleveland yielded similar results. When subjected to the same statistical analysis, Cleveland's data resulted in an equation Table 1. Index Properties of Soils at the Test Sites WATER CONTENTS (PERCENT) GRWATJON (PERCENT) LOCATION DEI'TH LlQUID!TY UNIFIW (METERS) NATURAL LIQUID PLASTICITY DEGREE OF INDE){ CLASSIFICATION SAND SILT CLAY -==-=------c:--cc:---::---''cc': ::<':O:: :C':CC'C'':' ~:----c:;----':_::':_' :c_::':: o.ooo; CM.ICO ':': W:':'::::l.l! OWEN CO.. ll.l JO ,5 CL 46 5 OWEN CO 12.2' 18.3! ,38 CL FAYETTE CO C CL ~ BOYD CO II CL J<J 41 4U LAWREI-:CE CO I 1. SM 6 CU 14 BOREHOLE I la LAWRENCE CO BOREHOLE BA. (.1 Jl J O.ll ML CL 45 UNIV. OF KY.' CAMPUS IJNIV. OF ){V... ]_].11 MH CH POST OFFICE POULTRY!'ARM KENTUCKY RIVER CL ML LOCK NO. '' LEXINGTON. KY... J,l -.14 CL Ml,, ' ~Y CLEVELAND Table 2. Summary of Triaxial and Dutch Cone Data TRIAXIAL DATA OUTCII CONE DATA SITE UUI(Ef]OLE NUMBER DEPTH (METERS) SHEAR STRENGTII PARAMHI'RS UNDRAINF.D SHEAR SOUNDING LOCATION i'riction Sl.HVf_ CON!' STRENGTII NI.IMBER IU'SISTANCE R~SISTANl'f. ;',, (OEORHS) (kg/em) (ki1:~ 1 J OWf.)l CO. 4.(, l.l U.2 CJ.].9.a 1.7-JI.J l.l J 1(> ll.j i ll.6 no ) Jl.l.41 O.O~l om I.SmfromBill <!.7 FAYICLH CO 4.tl l.j (>.1 6,7 ),{,. ~ , 4,(,.J,2 ll..l l om.5r U.JM I.OJ ,8<) 4. l. ~ o,v m W of Bll I l.j m IV. 1.2 m W, O.V ' of ~ll l l.ll 1.11 ].~9 l.lo UOYD CO. LAWRiiN['E ('. ' '.1... \ ~.l 7.6. ~ :!.1 J.7-4.J.1.-J.l JJ.O l1.4 ~~.8.6 o.r.o.6 O.. IJ.12 O.J4 l.ll 1.4! 1.46 OM l 1.6 m li nf UH I 4.lm\VofUlll l.lmsofuiim U m S of UH aa J.l 111 SE of Bll ~ l4.<j m S of BH II & U m S <>f 1111 IIA I.IB 1.$ O.'JI.1 JI.<J jc),).\<j,o 2

5 of fs = 'p Hoever, Cleveland reproduced in situ conditions in an unconsolidated, undrained triaxial test by applying stresses equal to the full overburden pressure to the sample. In this research, in situ conditions ere reproduced in a consolidated, undrained triaxial test by applying effective stresses equal to 3/4 of the overburden pressure. Combining data from this research ith Cleveland's data resulted in a regression equation of fs ' 1.24 Tf (see Figure 4). The difference in the mechanisms of failure should be considered in any discussion of shear strength and Dutch cone sleeve friction. In the triaxial test, or in situ, undrained shear strength is the shear stress on a soil-soil interface knon as the failure plane. This plane forms an oblique angle ith tbe vertical hich is usually unknon. Dutch cone sleeve friction, hoever, is the frictional resistance developed along a vertical steel-soil interface. This difference makes theoretical correlation of the to quantities extremely difficult. Therefore, empirical correlation seems to offer the best means of associating the to quantities. I, 1.24 T, - OWEN COUNTY D- LAWRENO< tounty A- BOYO COUNTY 'HTTE OOUNTY UNIV. OF I<Y CA~PUS CDCK#9 POULT' FAF,< No corrections ere applied to the Dutch cone sleeve friction values to account for the differences in soil type or conditions. Thus the correlations shon in Jligure 4 represent a ide variety of soil types and conditions of saturation and consolidation. In addition, experimental scatter may be expected in both triaxial and Dutch cone testing. Triaxial test scatter can be caused by disturbances during sampling and trimming of the specimen and vertical variation in the soils tested for a given set of triaxial dhta. In situ conditions ere duplicated in the triaxial test by isotropic consolidation of the specimen using a value of K equal to.62, Lateral in situ stresses are difficult, at best, to predict and most certainly varied for the soils tested. '. Figure 4. :c----fc- ;',---~--c,cc, SH~~~AJ;~~EN~J~T BY ( ~g/cml Relationship beteen Dutch Cone Sleeve Friction and Undrained Shear Strength Dutch cone soundings ere taken at various distances from the bore holes from hich the undisturbed samples ere taken. Any h1teral variation in soil properties could also lead to variations in sheur strengths, hich in turn could produce scatter unrelated to the test methods. CONCLUSIONS DISCUSSION The results of this study and the results of Cleveland (1971) and Wesley (1967) sho very close agreement. Shon in Figure 5 are the relations beteen friction sleeve resistance, fs, and undrained shear strength resulting from the three independent studies. In all cases, fs as found to be slightly higher than the undrained strength as measured by laboratory tests. Begemann initially set undrained shear strength as the upper limit for sleeve friction; hoever, Wesley attributed the higher values of fs to secondary loads (forces acting on the bevelled loer edge of the friction sleeve) and high penetration rate. ' E ' ~ 1. '' ' i u > ~ '' ' ' z.4 u ~ u >-., Figure 5. ~l / -- WESLEY j / /~CLEVELAND /;'! // ///.2. o.4 o.6 o.a 1.o 1.2 UNDRAINED SHEAR STRENGTH (Kg/cm 2 ) Comparison of Various Relationships Beteen Dutch Cone Sleeve Friction and Undrained Shear Strength / For a variety of cohesive soils that include residual silty clays, compacted embankments, and alluvial clayey silts, undrained shear strength as measured by triaxial tests as found to be approximately 8 percent of the friction sleeve resistance as measured by the Begemann friction sleeve cone penetrometer. Friction sleeve resistance provided a better correlation ith undrained shear strength than did cone resistance. This could be due in part to encountered rock fragments having less an effect on the friction sleeve r'esistance than on the cone penetration resistance. Unconsolidated-undrained and consolidated-undrained triaxial tests ere performed, In the former, the confining pressure as equal to the total overburden stress, and in the latter, the effective confining pressure as made equal to 75 percent of the mean effective principal stress. Both types of tests yielded approximately the same correlation, implying that the unconsolidated-undrained type of test is sufficient. Thus, a rough estimate of undrained shear strength may be obtained from friction sleeve resistance using the correlation developed herein, For more accurate determinations of in situ shear strength, it is recommended that correlations be established at a given site, REFERENCES BEGEMANN, H. K. S., Improved Method of Determining Resistance to Adhesion by Sounding through a Loose Sleeve Placed behind the Cone. Proceedings, International Conference on Soil Mechanics and Foundation Engineering, Zurich, Vol I, pp BEGEMANN, H. K. S., ]965. The Friction Jacket Cone as an Aid in Determining the Soil Profile. Proceedings, International Conference on Soil Mechanics and Foundation Engineering, Montreal, Vol 1, pp BISHOP, A. W. and HENKEL, D. J., The Measurement of Soil Properties in the Triaxial Test. Edard Arnold Publishers, Ltd., London, Table 2, p 143. CLEVELAND, E. P., Use of the Dutch Cone Penetration Test for Soil Exploration in Kentucky. Thesis presented to the University of Kentucky in partial fulfillment of the requirements for the degree of Master of Science. 3

6 DRENEVICI-I, V. P., Use of Conventional Boring Rigs for Penetration Testing. Proceedings, European Symposium on Penetration Testing, Stockholm, Vol 2. LAMBE, W. T., Methods of Estimating Settlement. Journal of the Soil Mechanics and Foundations Division, ASCE, Vol 9, No. SM5, pp LAMBE, W. T. and WHITMAN, R. V., Soil Mechanics. John Wiley and Sons, Inc., Ne York, Figure 2, p 3. SANGLERAT, G., The Penetrometer and Soil Exploration. American Elsevier Publishing Co., Inc., Ne York. SIMONS, N. W., 196. The Effect of Overconsolidation on the Shear Strength Characteristics of an Undisturbed Oslo Clay. Proceedings, Research Conference on Shear Strength of Cohesive Soils, ASCE, June 196. THOMAS, D., Static Penetration Tests in London Clay. Geotechnique, Vol 15, No.2, pp TOMLINSON, M. 1., The Adhesion of Piles Driven in Clay Soils. Proceedings, International Conference on Soil Mechanics and Foundation Engineering, London, Vol 2, pp VESIC, A. B., Discussion, Proceedings, International Conference on Soil Mechanics and Foundation Engineering, Mexico, Vol 3, pp WESLEY, L. D., The Dutch Penetrometer and Its Use in Indonesia. Proceedings, Southeast Asian Regional Conference of Soil Engineering, Bangkok, pp